1. Field of the Invention
The invention relates to an implant having an impedance detection unit for determining a transthoracic impedance course and an impedance analysis unit, which is connected to the impedance detection unit and analyzes the impedance course, as well as a system and a method for analyzing the impedance course. The analysis of the impedance course is performed in each case with the goal of recognizing a decompensation as early and precisely as possible.
2. Description of the Related Art
In patients who suffer from cardiac insufficiency, acute worsening of their state may occur at irregular intervals, connected with a complete collapse of performance capability, respiratory distress, and fear of suffocation. In this case, immediate hospitalization with emergency treatment must be performed.
These events are caused by a so-called “decompensation”, in which the heart is no longer capable of providing a sufficient pumping delivery rate. This event gives notice several weeks beforehand by a (strengthening) rise of the pressure in the pulmonary circulation, and an increasing storage of water in the pulmonary tissue connected thereto, which is not typically perceived in a timely manner by the patient, however.
The occurrence of a decompensation, connected with a great strain of the patient and of the cardiopulmonary system itself, significantly worsens the clinical picture of the cardiac insufficiency. The life expectancy of the patient may be significantly increased by timely recognition of looming decompensations. The costs of medical care may be reduced by the possible avoidance of the hospitalization necessary due to a critical state of the patient.
A device which generates a warning upon initiation of a decompensation in a timely manner before the acute phase is required for this purpose. Appropriate therapy measures may then be introduced early and a hospitalization may be prevented.
Patients who suffer from heart failure are increasingly provided with an implantable defibrillator (ICD), because parts of this patient group also have an increased risk for life-threatening tachyarrhythmias.
The ICDs used are implemented as single-chamber or dual-chamber systems. Approximately ⅔ of all patients having heart failure sufferer from systolic heart failure, in which the efficiency of the blood ejection is greatly worsened (e.g., caused by a left bundle branch block). Such patients are currently increasingly treated using an implant, which reproduces the synchronicity of the contraction of the left and right heart by synchronous stimulation of the right and left heart. They thus receive a so-called cardiac resynchronization therapy (CRT), in which a pacemaker or ICD receives separate electrodes to the right and left ventricles (via the coronary sinus).
A physical parameter, using which the increasing accumulation of liquid in the thorax may be ascertained, is the transthoracic electrical impedance. If the liquid content in the pulmonary tissue increases, the measured impedance drops. With the aid of an implant and the implanted electrodes, this transthoracic impedance may be measured easily. A decompensation may thus be recognized early before the acute phase and thus the patient or physician may be warned to then initiate therapeutic measures.
Devices which measure the transthoracic impedance between the implant housing and/or one or more cardiac electrodes to detect a liquid accumulation in the lungs are known (U.S. Pat. No. 5,957,861, U.S. Pat. No. 6,076,015, U.S. Pat. No. 6,454,719, US 2006/0041280, US 2006/0258952, US 2006/0264776). In general, the impedance values are averaged over a longer period of time (e.g., over 24 hours), to compensate for the impedance variations caused by the cardiac and respiratory cycles and circadian oscillations. These averaged values are used as the basis for the early recognition of looming pulmonary edemas; see Yu C M, Wang L, Chau E, Chan R H, Kong S L, Tang M O, Christensen J, Stadler R W, Lau C P. “Intrathoracic impedance monitoring in patients with heart failure: correlation with fluid status and feasibility of early warning preceding hospitalization.” Circulation 2005; 112(6):841-8.
Secondary effects which are not caused by the occurrence of a pulmonary edema may also strongly influence the transthoracic impedance and simulate or conceal a liquid accumulation. These interferences must be compensated for in their influence on the detection of an occurring pulmonary edema.
One of these interferences may arise due to oscillations of the blood conductivity, caused by a changing hematocrit or the varying electrolyte content in the blood, for example.
Systems which determine the blood conductivity and use it for correction of the transthoracic impedance to reduce the influence of the blood conductivity have been described (US 2006/0041280, US 2006/0258952, US 2006/0264776).
The impedances of lung tissue and blood have a differing frequency characteristic. A system has been described which performs the impedance measurement at various frequencies for this reason, to minimize the influence of the blood conductivity (EP 1 665 983).
Changes of the body position (e.g., standing up or lying down) result in a liquid redistribution in the body and thus a temporary change of the liquid content in the lungs. The changes in the liquid content are reproduced in principle by the transthoracic impedance and possibly evaluated as an occurring pulmonary edema. For this reason, systems have been described which detect the body position or its change and incorporate it in the evaluation of the transthoracic impedance (US 2006/0041280, US 2006/0258952, US 2006/0264776). Further interferences may arise due to liquid accumulations in the implant pocket or migration of the implant.
All of these solutions have the disadvantage that additional sensors or additional measurements are necessary. The energy required for this purpose may result in a significant decrease of the service life in long-term implants in particular.
Systems have also been described which monitor the respiratory activity itself to conclude a worsening of the clinical picture from the ascertained respiratory frequency or the respiratory rhythm (e.g., shortness of breath, Cheyne-Stokes breathing, sleep apnea) (U.S. Pat. No. 5,876,353, U.S. Pat. No. 5,957,861, U.S. Pat. No. 6,076,015, U.S. Pat. No. 6,449,509, U.S. Pat. No. 6,454,719, US 2006/0258952).
However, the respiratory activity is first noticeably influenced in a relatively late stage of the pulmonary edema, see Zipes, D. P. et al. [ed.]: Braunwald's Heart Disease; Elsevier, 2005. In addition, however, respiration is also influenced by many other factors, such as physical stress, speaking, and general state of health (NYHA). The decisive early recognition of a pulmonary edema solely from the respiration is thus very susceptible to error. Thus, for example, the dependence of the impedance on the respiration is also exploited to determine the respiratory minute volume, which is used to estimate the metabolic load and employed to activate a frequency-adaptive pacemaker (U.S. Pat. No. 6,076,015, U.S. Pat. No. 6,449,509).
Furthermore, methods have been described which determine hemodynamic variables via an intracardially measured impedance and derive a change of the clinical picture from their change. The dynamics of the heartbeat itself are determined and oscillations which are caused by respiration and by other influences are removed by averaging, for example (Zima, E., et al. “Determination of left ventricular volume changes by intracardiac conductance using a biventricular electrode configuration.” Europace 8.7 (2006): 537-44; Stahl, C., et al. “Intracardiac Impedance Monitors Hemodynamic Deterioration in a Chronic Heart Failure Pig Model.” J. Cardiovasc. Electrophysiol. 18 (2007): 985-90; EP 1 510 173).
Systems are also known which combine multiple different parameters to judge the clinical picture better and be able to compensate for the uncertainties of an individual parameter (U.S. Pat. No. 5,876,353, U.S. Pat. No. 5,957,861, US 2006/0258952, US 2006/0264776).
The known solutions have various disadvantages.
A decisive disadvantage of the known solutions is based in the fact that the transthoracic electrical impedance is not only influenced by the increasing liquid accumulation in the lungs, but rather also by many other factors. Due to the influences of the secondary factors, the sensitivity and specificity during the lung water detection are reduced. Therefore, the secondary influencing factors must be compensated for by additional parameters or additional measured variables, partially also from other sensors (e.g., body position, intracardial pressure sensors). Additional measurements increase the effort and the power consumption. The service life of the system is decreased by the increased power consumption, above all in long-term implants. The known respiration parameters are also dependent on many other factors and less specific when taken alone. The use of additional parameters is thus also necessary for this purpose.